1. Field of the Invention
This invention relates to improvements in an internal combustion engine, and more particularly to the improvements in control of an amount of intake air inducted into the engine and an amount of EGR (Exhaust Gas Recirculation) gas recirculated from an exhaust gas passage back to an intake air passage of the engine.
2. Description of the Related Art
Hitherto a variety of EGR control techniques have been proposed and put into practice to effectively control an EGR rate in accordance with engine operating conditions, as disclosed, for example, in Japanese Patent Provisional Publication No. 53-140420. These techniques are basically arranged as follows: A basic opening area of an EGR valve is determined by multiplying an opening area of a throttle valve by a target EGR rate. The basic opening area of the EGR valve is corrected by an amount corresponding to a change in an EGR gas flow amount due to exhaust gas pressure and temperature, thereby setting a target value of the opening area of the EGR valve.
However, drawbacks have been encountered in such conventional EGR control techniques, in which a condition (such as pressure and temperature) within the intake system is changed by EGR gas. Consequently, the amount of intake air is unavoidably changed with an EGR rate even when the opening area of the throttle valve is the same, and therefore a required EGR rate cannot be realized at a high precision while causing a stepwise torque change in the engine. Additionally, particularly in a cylinder direct injection spark-ignition engine which has been recently promoted in development and makes so-called stratified charge combustion in a cylinder, it is required to largely increase the EGR rate in order to cope with an increase in NOx emission due to engine operation on an extremely lean air-fuel mixture. In this case, control error in EGR rate largely affects operation of the engine and cannot be ignored, although the control error is small in conventional engines having an EGR rate lower than 10%.
Furthermore, in engines where so-called torque demand control is accomplished, a throttle valve is electronically controlled in order to introduce intake air in an amount required corresponding to a target torque. Also in such engines, the amount of intake air is unavoidably changed with the EGR rate thereby making it difficult to appropriately control the relationship between the target torque and a target amount of the intake air.